Suspected changes in tooth sensitivity to pain with ageing may be partly explained by the number of axons entering the teeth. After vitalometer testing, twenty-two mandibular first premolars (eleven newly erupted and eleven from adults) were extracted, immediately cracked, immersed in glutaraldehyde and prepared for electron microscopy. From juxta-apical cross sections, montages were constructed. Number of myelinated axons was significantly greater for teeth from older persons. Myelinated axon development appears incomplete prior to apex closure and continues for some time after. Number of unmyelinated axons was similar for the two groups but with marked variability for this parameter. From size spectra, myelinated axon circumference was greater for single unmyelinated axons in newly erupted teeth. Groups of unmyelinated axons were larger in newly erupted than older teeth. From vitalometer tests, threshold sensitivity to electrical stimulants decreased with apical closure and tended to further decrease gradually with ageing. The negative correlation of myelinated axon numbers with vitalometer readings was statistically significant (r equals No significant correlation found for unmyelinated axon number and vitalometer readings). Several newly erupted teeth were unresponsive to vitalometer testing but had more axons than some other teeth that did respond.